There are significant challenges associated with designing clot removal systems that can deliver high levels of performance:
There are a number of access challenges that make it difficult to deliver devices. In cases where access involves navigating the aortic arch (such as coronary or cerebral blockages) the configuration of the arch in some patients makes it difficult to position a guide catheter. These difficult arch configurations are classified as either type 2 or type 3 aortic arches with type 3 arches presenting the most difficulty. The tortuosity challenge is even more severe in the arteries approaching the brain. For example it is not unusual at the distal end of the internal carotid artery that the device will have to navigate a vessel segment with a 180° bend, a 90° bend and a 360° bend in quick succession over a few centimeters of vessel. In the case of pulmonary embolisms, access may be gained through the venous system and then through the right atrium and ventricle of the heart. The right ventricular outflow tract and pulmonary arteries are delicate vessels that can easily be damaged by inflexible or high profile devices. For these reasons it is desirable that the clot retrieval device be compatible with as low profile and flexible access and support catheters as possible.
The vasculature in the area in which the clot may be lodged is often fragile and delicate. For example neurovascular vessels are more fragile than similarly sized vessels in other parts of the body and are in a soft tissue bed. Excessive tensile forces applied on these vessels could result in perforations and haemorrhage. Pulmonary vessels are larger than those of the cerebral vasculature, but are also delicate in nature, particularly those more distal vessels.
The clot may comprise any of a range of morphologies and consistencies. Long strands of softer clot material may tend to lodge at bifurcations or trifurcations, resulting in multiple vessels being simultaneously occluded over significant lengths. More mature and organized clot material is likely to be less compressible than softer fresher clot, and under the action of blood pressure it may distend the compliant vessel in which it is lodged. Furthermore the inventors have discovered that the properties of the clot may be significantly changed by the action of the devices interacting with it. In particular compression of blood clot causes dehydration of the clot and results in a dramatic increase in both clot stiffness and coefficient of friction.
The clots may not only range in shape and consistency, but also may vary greatly in length, even in any one given area of the anatomy. For example clots occluding the middle cerebral artery of an ischemic stroke patient may range from just a few millimeters to several centimeters in length.
In the case of an intracranial occlusion a variety of access routes are possible with known devices, including a direct stick into the carotid artery, a brachial approach, or a femoral access. Once access has been gained to the arterial system using conventional and well understood techniques, a guide catheter or long sheath is typically placed as close to the occlusive clot as practical. For example, In the case of a middle cerebral artery occlusion the guide catheter might be placed in the internal carotid artery proximal of the carotid siphon. A microcatheter is then advanced across clot, typically with the aid of a guidewire. In some cases an additional catheter (which may be known as a Distal Access Catheter or DAC) may be used in a triaxial system such that the microcatheter is advanced through the DAC, which is in turn advanced through the guide catheter or long sheath. Once the microcatheter tip has been advanced across and distal of the clot the guidewire is removed and the clot retrieval device is advanced through the microcatheter until it reaches its distal end. The microcatheter is then retracted, allowing the clot retrieval device to expand within and on either side of the clot.
A particular problem arises with known systems because of the multiple catheters/shafts required to be in place at various stages during the procedure. For example in many cases it is desirable to be able to aspirate (apply a suction force) through the guide/sheath or DAC to assist in the withdrawal of the clot. The effectiveness of this aspiration can be hindered by the presence of catheter shafts within the aspiration lumen, and it is therefore sometimes desirable to be able to remove the microcatheter prior to aspiration and clot retrieval. The vessels through which the catheters are passed are very narrow and in most cases very tortuous. Thus, the anatomy presents major challenges to removing or advancing further devices or catheters that may be required during a procedure, as clot retrieval device shafts are typically not exchange length.
In general, there is a need to provide a clot retrieval system which provides the required flexibility to a physician to deal with a wide range of clots, often in an emergency situation.